The high consumption of maize by the human population in a
number of countries in Latin America and Africa and the
well-established lysine and tryptophan deficiencies in maize
protein motivated the search for a maize kernel with higher
concentrations of these essential amino acids in its protein. The
possibility of finding better varieties of maize appeared
feasible on the basis of three facts. One was that by selection,
oil content in the maize kernel could be increased from about 4
to 15 percent (Dudley and Lambert, 1969). This increase was
obtained by increasing the size of the germ, the part of the
kernel where the oil is concentrated. The same researchers showed
that it was possible to increase total protein content from about
6 to 18 percent by increasing the prolamine (zein) fraction in
maize endosperm. The third finding was the wide variability in
lysine content reported among varieties and selections of maize.

TABLE 35 - Summary of the nitrogen balances of
children fed whole milk and opaque-2 maize (nitrogen values in
mg/kg/day)

Treatment

Nitrogen intake

Faecal nitrogen

Urinary nitrogen

Nitrogen absorbed

Nitrogen retained

% N intake absorbed

% N intake retained

1.8 g protein/kg/day

Milk

277

52

157

225

68

81.2

24.5

Opaque-2

295

72

140

223

83

75.6

28.1

Milk

271

42

152

229

77

84.5

28.4

1.5 g protein/kg/day

Milk

187

31

88

156

68

83.4

36.4

Opaque-2

238

68

108

170

62

71.4

26.0

Milk

190

34

108

156

48

82.1

25.3

Source: Bressani, Alvarado and Viteri, 1969

The search for a high quality protein maize succeeded when
Mertz, Bates and Nelson (1964) announced their discovery that the
opaque-2 gene used as a marker in maize breeding significantly
increased lysine and tryptophan in the cereal protein.

Results from initial alkaline processing studies of opaque-2
maize (cultivated in Indiana, United States, in 1965) showed that
the process did not induce significant nutritional changes in the
dough or in the tortillas, as concluded from chemical data and
biological trials with rats.

The protein quality of alkali-processed opaque-2 maize was
evaluated in children using the nitrogen balance index (the
relationship between nitrogen absorption and retention). Six
healthy children were used in two studies. The average nitrogen
balances, at protein intake levels of 1.8 and 1.5 g per kg body
weight per day, are presented in Table 35 (Bressani, Alvarado and
Viteri, 1969). As can be observed, there were no significant
differences in nitrogen retention among the children fed the
diets based on milk and on alkali-processed opaque-2 maize when
the level of protein intake was 1.8 g per kg per day. The data
demonstrate differences in nitrogen absorption.

The apparent protein digestibility for processed opaque-2
maize averaged 73.5 percent in these studies. Based on faecal
metabolic nitrogen determined in the children, true protein
digestibility was 83.8 percent. From these results, it was
concluded that the quantities of opaque-2 maize ingested by the
children were 16.3 to 16.7 g and 12.9 to 14.5 g per kg body
weight to take in 1.8 and 1.5 g protein per kg per day,
respectively. These figures are equivalent to a total maize
intake of 140 to 227 g per day, amounts similar to those commonly
ingested by children in Guatemala.

With the data obtained in this study and data on urinary
endogenous nitrogen, the relationship between nitrogen absorption
and retention from milk and from opaque-2 maize was calculated.
This nitrogen balance index constitutes a good measure for the
biological value of proteins. The index was 0.80 for milk and
0.72 for opaque-2 maize, establishing then that the protein value
of opaque-2 maize is equivalent to 90 percent of the biological
value of milk. When the figure for true digestibility was used,
the biological value of opaque-2 maize protein was calculated to
be 87.1 percent. The figures also indicated that 90 mg of
nitrogen must be absorbed from opaque-2 maize to obtain a
nitrogen equilibrium.

For comparative purposes the same type of analysis was carried
out for common maize in children (Scrimshaw et al., 1958;
Bressani et al., 1958, 1963). Data on the nitrogen balance index
were obtained from various studies in which children were fed
with maize proteins as the only protein source in their diet. The
biological value calculated was 32 percent. These data
demonstrated again the low quality of common maize protein.

The difference between the nutritive value of opaque-2 maize
protein and that of common maize is clearly shown in Figure 2,
obtained from data in the studies described above. This figure
shows the nitrogen retention in groups of children fed
exclusively with opaque-2 and in others fed with common maize, in
both cases at different protein intake levels. The
supplementation effect of lysine and tryptophan on common maize
is also shown. Even at intakes of 400 or 500 g of common maize,
nitrogen retention is quite low, and this decreases to lower
levels when the intake is reduced to200 or 300 g per day. With
opaque-2, on the contrary, intakes of 140 or 230 g per child per
day induced positive retentions that exceeded even those obtained
with common maize supplemented with lysine and tryptophan. This
suggests that it may be necessary to supplement common maize with
other amino acids to make it comparable in protein value to
opaque-2 maize.

The difference between opaque-2, common maize and the latter
supplemented with lysine and tryptophan can be attributed to the
better essential amino acid pattern found in opaque-2 maize,
since the digestibility of the three is essentially the same. QPM
maize also has a lower leucine content, implicated in the low
nutritional value of maize.

The information presented clearly indicates the superiority of
opaque-2 maize protein to that of common maize, a fact that is of
great importance for populations consuming large quantities of
maize as part of their habitual diet.

In a study by Luna-Jaspe, Parra and Serrano (1971) the
nitrogen retention of common maize, Colombian opaque-2 maize (ICA
H-208) and milk was compared in three children aged 24 to 29
months and weighing 5.9 to 10.1 kg. The protein and calorie
intakes were approximately 1 g and 100 calories per kg body
weight daily. Nitrogen retention was negative when the children
received the opaque-2 maize. Common maize, however, gave an even
lower or more negative figure. When milk was given, one child
showed a negative balance and the other two a positive one, with
the average balance on the positive side.

The authors indicated that the apparent protein digestibility
of common maize was 61.5 percent, opaque-2 maize 57.9 percent and
milk 66.4 percent. They also concluded that opaque-2 maize is of
a higher nutritional value than common maize. They pointed out,
however, that its use for young children with a rapid growth rate
should be carefully controlled, and they could not recommend it
as the main source of daily protein intake.

The results of these investigators are in agreement with those
reported by other workers (Bressani, Alvarado and Viteri, 1969).
The latter found that with 90 mg N absorbed per kg body weight
per day, nitrogen equilibrium was obtained. The investigators in
Colombia found that 90 mg absorbed nitrogen yielded a relatively
low negative retention, while 100 mg absorbed nitrogen yielded
nitrogen equilibrium. The differences between the results were
not significant, and they could be explained by the age of the
children, who were younger in the Colombian study and had lower
body weight than the subjects used in the 1969 study. A more
important factor was the lower protein intake. In any case, the
data suggest that a minimum daily intake of approximately 125 g
of opaque-2 maize might guarantee nitrogen equilibrium. This
could not be obtained by using even twice the amount of common
maize.

Similar studies were conducted by Pradilla et al. (1973) using
the same variety of maize but with the opaque-2 gene (H-208
opaque). A crystalline endosperm containing the opaque-2 gene was
also tested. The results are shown in Table 36, in which similar
values may be observed for digestibility, biological value and
nitrogen retention for the two maize selections containing the
opaque-2 gene. These values were slightly lower than those for
casein but significantly higher than values for common maize. In
more recent studies Graham et al. (1989) evaluated QPM Nutricta,
a maize variety containing the opaque-2 gene. This maize is high
yielding, has a hard endosperm and contains high levels of lysine
and tryptophan, although not as high as those in the original
opaque-2 maize first studied. These authors used six male
children aged 7.9 to 18.5 months who were recovering from
malnutrition. Common maize and QPM as well as a casein diet were
fed to provide 6.4 percent of the calories as protein. Total
energy intake was approximately 125 kcal per kg per day, which
was calculated to support weight and growth at previously
established rates. The nitrogen balance results are shown in
Table 37. Nitrogen absorption from QPM and common maize was 70
and 69 percent respectively, and 82 percent from casein. Nitrogen
retention as a percentage of intake was 32 percent for QPM as
compared with 41 percent for casein and 22 percent for common
maize. These results, like others previously reported, confirm
the great superiority of opaque2 maize to common maize as food
for children.

TABLE 36 - Comparative nitrogen balances in children
fed QPM and common maize

Graham et al. (1980) and Graham, Placko and MacLean (1980)
also reported on studies of eight convalescent malnourished
children, 10 to 25 months of age, who were fed opaque-2 and
sugary-2 opaque-2 endosperm and the whole kernel. Protein was fed
so as to provide 6.4 percent of total calories, and the diets
provided 100 to 125 kcal per kg body weight per day. The results
showed an apparent N retention from the endosperm meal lower than
that from the whole kernel meals, and both were lower than from
casein. The difference between whole kernel and endosperm
nitrogen retentions can probably be attributed to the amino acids
contributed by the germ. The same researchers reported on
plasma-free amino acids in the studies described above and
concluded that the types of maize tested were possibly limiting
in lysine tryptophan and isoleucine.

These authors also reported that for the children to match N
retention from casein, presumably equal to the requirement, they
would have to consume 203.9, 148 or 122.5 percent of their energy
requirements as common, opaque2 or sugary-2 opaque-2 endosperm
meals, which is impossible. For whole meals, they would have to
consume 108.2, 90.3 or 84.2 percent of their energy as common,
opaque-2 or sugary-2 opaque-2 maize.

Growth studies of children fed QPM have been conducted by
various workers, among them Amorin (1972) and Valverde et al.
(1981). In all reports, QPM was significantly superior to common
maize and only slightly below the growth response observed when
milk was fed.

Graham et al. (1989) stated: "To anyone familiar with the
nutritional problems of weaned infants and small children in the
developing countries of the world, and with the fact that
millions of them depend on maize for most of their dietary
energy, nitrogen and essential amino acids, the potential
advantages of quality protein maize are enormous. To assume that
these children will always be given a complementary source of
nitrogen and amino acids is a cruel delusion."

Human adults

Two studies evaluating the protein quality of opaque-2 maize
for human adults have been published. In the first, Clark et al.
(1967) used ten university students as subjects in two
experiments. The maize utilized was finely ground and included
the whole grain. It contained 11 to 12 percent protein, 4.65 g
lysine per 16 g N and 1.38 g tryptophan per 16 g N. values
similar to those of the opaque-2 maize used in the study of
children by Bressani, Alvarado and Viteri (1969). The maize was
given in quantities of 300, 250, 200 and 150 g per day, which
provided 5.58, 4.65, 3.72 and 2.79 g nitrogen per individual per
day. The results of one experiment are shown in Table 38. All the
individuals were in positive balance with an intake of 300 g of
the maize and all of them were in equilibrium when they were
administered 250 g. The 200 and 150 g levels resulted in a
negative balance. With these data the regression equation between
nitrogen balance and maize consumed was calculated. On the
average, nitrogen equilibrium was obtained with an intake of 230
g.

The same authors studied the effect of lysine or tryptophan
supplementation alone. Only one subject showed improved nitrogen
retention. The addition of methionine did not induce any change.
This indicated that the protein of opaque2 maize was not
deficient in these three amino acids for adult human subjects.
Similar results were reported by Clark et al. (1977) for adult
human subjects fed QPM and sugary-2 opaque-2 maize.

Unfortunately no studies have been done on adult human
subjects comparing opaque-2 and common maize in the same study.
The protein quality of common maize has, however, been evaluated
in human adults by Kies, Williams and Fox (1965). In one study
ten subjects were fed degermed maize to provide nitrogen intakes
of 4, 6 and 8 g per day. The results clearly indicated that when
the degermed maize provided 4 and 6 g of nitrogen, the average
nitrogen balance was negative. When the intake increased to 8 g
of nitrogen per day, the balance became positive. The regression
between nitrogen intake and nitrogen retained was calculated.
From the equation it was calculated that 6.9 g of degermed maize
nitrogen was necessary to give nitrogen equilibrium. The
regression coefficient, multiplied by 100 and divided by the
protein digestibility, gives the biological value of the protein.
In the present case this value was 46.5 percent.

Based on 8.0 g protein per 100 g degermed maize, an intake of
6.9 g nitrogen is equivalent to 539 g maize. This figure is close
to levels of maize consumed by adults in Mexico, Guatemala and El
Salvador.

In the study described above lysine and tryptophan added alone
did not produce changes in average nitrogen retention. When both
amino acids were added, however, nitrogen retention increased -
not necessarily because of the higher amount of nitrogen being
administered with the addition of these two amino acids. This
possibility may be discarded in view of the response obtained
when non-specific nitrogen was added. These data demonstrate that
the common maize protein is deficient in lysine and tryptophan
for adult humans, as it is for children (see above in this
chapter).

The results of these studies of amino acid intake from QPM and
common maize (Clark et al., 1967; Kies, Williams and Fox, 1965)
are compared in Table 39. As shown earlier in this chapter, twice
as much common maize is necessary to obtain nitrogen equilibrium
in adults. This is equivalent to a protein intake of
approximately 1.6 times more from common maize than from
opaque-2. EAA intake follows the same trend as total nitrogen
intake.

Using a biological value of 82 percent for opaque-2, of the 28
g ingested about 23 g are retained, which is the approximate
amount (21 g) retained from common maize, which has a biological
value of 46.5 percent. These data indicate the great losses of
nitrogen occurring with common maize. With the exception of
lysine and tryptophan, common maize provides a greater quantify
of essential amino acids. They are, however, a load the body has
to discard, a load that is greater in the cases of leucine,
tyrosine and valine. The physiological cost of metabolizing these
unnecessary amino acids is unknown, but it should be estimated.

Furthermore, the amino acid intake pattern is unbalanced,
which may be an additional reason for the poor biological value
of the common maize protein. Another method of analysing intake
of individual amino acids is to express it as a percentage of the
total amino acid intake, a calculation which magnifies the
deficiency in lysine and tryptophan in common maize and also
indicates the excess of other amino acids. This information, in
reference to adults as well as children, demonstrates once more
the excellent quality of opaque-2 maize protein and the poor
quality of common maize protein.

No direct comparative studies are available on the
digestibility and biological value of common and opaque-2 maize
proteins, so to make a comparison between them the studies of
common maize by Truswell and Brock (1961, 1962) and of opaque-2
maize by Young et al. (1971) will be used. In one of the studies
conducted by Truswell and Brock (1962), the experimental subjects
received 90 percent of their nitrogen intake from maize and 10
percent from other foods. A positive nitrogen balance was
obtained when the nitrogen intake was more than 7 g per day,
although great variability was found as in other studies. The
authors calculated the biological value, which averaged 45
percent at a high intake level and 57 percent at a lower level of
nitrogen intake. The difference was to be expected, since the
biological value of a protein depends on the level of protein
intake. Since all the experimental subjects showed a positive
nitrogen balance when the intake was high, the authors concluded
that the biological value of maize is close to the 57 percent
figure. Similar results were found by Young et al. (1971).
Truswell and Brock (1961) also found that in adult human subjects
fed maize, the addition of lysine tryptophan and isoleucine
increased nitrogen balance from 0.475 to 0.953 g N per day in one
study and from 0.538 to 1.035 g N per day in a second study. The
flour fed was degermed maize flour, in which deficiencies are
more apparent.

The biological value of opaque-2 maize protein was studied by
Young et al. (1971). Egg protein was used as reference, fed at
intakes of 2.64 to 3.95 g nitrogen per day. The authors
calculated true protein digestibility and biological value from
the faecal metabolic nitrogen and urinary endogenous nitrogen.
The protein digestibility of opaque-2 maize protein varied from
67 to 106 percent, with an average for the eight individuals in
the study of 92 percent, while the variability for egg protein
was from 78 to 103 percept with an average of 96 percent. The
average biological value for opaque-2 maize was 80 percent, and
for egg the average was 96 percent.

Practical significance of protein evaluation of
opaque-2 maize

The evidence presented from studies in both children and
adults clearly indicates the superiority of opaque-2 maize over
common maize. In spite of this, of the maize-consuming countries
only Colombia and Guatemala have made efforts during the last few
years to introduce this superior maize into agricultural
production systems. The reasons are not clear, since agronomic
studies conducted in a number of locations have shown that there
are no differences between QPM and common maize in cultural
practices, yield per unit of land and physical quality of grain.
Furthermore, the plants look alike; QPM kernels are crystalline
and grain yields are comparable to those of common maize. These
factors are perhaps more important to growers than the
nutritional advantages offered by QPM.

Energy content is alike in both types of maize, but the
protein content of QPM is higher and is better utilized because
of its better essential amino acid balance. The protein value of
opaque-2 maize, however, can be analysed from other points as
well. The information in Table 39 could be used to decide whether
to introduce the opaque-2 maize varieties in grain-consuming
countries.

It has been established that the intake of both types of maize
as well as their nitrogen content (protein) are alike, but their
digestibility percentages are very different: of 48 g of nitrogen
intake from common maize, only 39.4 g are absorbed and 8.6 g are
lost in the faeces. In the opaque-2 maize, of the 48 g of
nitrogen intake, 44.2 g are absorbed and 3.8 g are lost in
faeces.

The factor that should be considered, then, is the biological
value, which is defined as the amount of absorbed nitrogen needed
to provide the necessary amino acids for the different metabolic
functions. The biological value of common maize is 45 percent;
from the 39.4 g absorbed, 17.7 g are retained and 21.7 g are
excreted. In opaque-2 maize the biological value of the protein
is 80 percent; of the 44.2 g of absorbed nitrogen, 35.4 g are
retained and 8.8 g are excreted. The total amount of nitrogen
lost when common maize is consumed equals 30.3 g, while only 12.6
g are lost when the same amount of opaque-2 maize protein is
consumed. In other words, only 37 percent of the common maize
intake is utilized, compared to 74 percent from opaque-2 maize.
The production and consumption of QPM in maize-eating countries
would therefore have a significant beneficial effect on the
nutritional state of populations, with important economic
implications from the better use of what is produced and
consumed.